Polycrystalline particles typically have a crystallite size above about 25 nanometers and a particle size above about 50 nanometers. In the prior art, others have defined materials having a particle size between about 1-100 nanometers as nanophase, polycrystalline or nanocrystalline materials. Polycrystalline particles have demonstrated unique chemical and physical properties, such as high reactivity, enhanced infrared absorption, novel electronic properties, magnetic properties, and improved hardness and ductility. From a practical standpoint, polycrystalline materials also have potential applications in advanced information and energy technologies, as well as military applications.
Several techniques are known in the art for forming polycrystalline particles. Two general categories for polycrystalline particle processing are: 1) aqueous processing and 2) gas phase processing. Aqueous processing includes techniques such as spray conversion pyrolysis, sol gel deposition, and electrodeposition. Gas phase processing may incorporate techniques such as sputtering, laser ablation, ohmic evaporation, high-energy milling, chemical vapor condensation, and gas phase condensation. Each of the aforementioned techniques have their unique characteristics, however, each is identified by the basic processes of nucleation and growth of a crystalline structure.
In the field of gas phase condensation for the preparation of polycrystalline particles, the basic processes include evaporation of a source material or materials, nucleation of the material, and growth within a vapor phase. Typically polycrystalline materials which are produced by gas phase condensation may be formed within an inert atmosphere or in an atmosphere consisting of a mixture of inert gases and reactive gases. Within the field of gas phase condensation, there are various techniques for vaporizing materials. One such technique is electrical joule heating, also known as ohmic heating, wherein the material to be vaporized is placed in a refractory crucible and upon the application of sufficient electrical current the crucible is heated and the material is vaporized.
One method of ohmic evaporation is disclosed in U.S. Pat. No. 5,128,081 to Siegel, et al., for "METHOD OF MAKING NANOCRYSTALLINE ALPHA ALUMINA." However, one disadvantage of using ohmic evaporation, such as the technique disclosed in the '081 patent, is the temperature limitation because the source material is heated indirectly. Therefore, high melting temperature materials such as nickel are difficult to prepare according to the teachings of Siegel. There are other shortcomings associated with resistive heating for evaporation, such as a limited heat conductance rate and poor efficiency. As a result, electrical resist heating suffers from a low production rate. Furthermore, contamination of the evaporated species from the heating element and crucible materials of the heating apparatus is also a problem.
Another method known in the art for evaporation of materials for gas phase condensation is electron beam evaporation. One such method for electron beam evaporation used in gas phase condensation processing of polycrystalline particles is disclosed in U.S. Pat. No. 5,728,195 to Eastman, et. al. for "METHOD FOR PRODUCING NANOCRYSTALLINE MULTICOMPONENT AND MULTIPHASE MATERIALS," Although the '195 patent discloses a good method for evaporating different sources materials to form "nanocrystalline" particles, this approach does have its disadvantages. Electron beam techniques involve sophisticated equipment that requires a differential vacuum pumping system and a delicate electron optical system. Furthermore, electron beam evaporation is not a continuous process, which prevents it from being a suitable industry technique. Furthermore, the electron beam itself emits harmful, high-energy radiation. Another application of electron beam evaporation is disclosed in U.S. Pat. No. 4,448,802 to Buhl, et. al. for "METHOD AND APPARATUS FOR EVAPORATING MATERIAL UNDER VACUUM USING BOTH AN ARC DISCHARGE AND ELECTRON BEAM." The '802 patent discloses a technique for evaporating materials by incorporating energy from an electron gun along with a low-voltage arc discharge. Although this is an interesting approach, this device suffers from the complexities discussed regarding the '195 patent along with the additional complexity of incorporating an electron gun with an arc discharge technique for evaporation.
Another technique available for evaporation of materials for gas phase condensation is known as arc discharge, and is also referred to as arc plasma, or arc evaporation. Arc plasma is a good technique for evaporating high melting point and low vapor pressure transition metals. One apparatus for arc evaporation of materials is disclosed in U.S. Pat. No. 4,732,369 to Araya, et al. for "ARC APPARATUS FOR PRODUCING ULTRAFINE PARTICLES." Araya discloses an apparatus for forming ultrafine particles by arc evaporation that is characterized by forming a magnetic blow to an electric arc by inclining an electrode to the material to be evaporated, causing an unbalance in electromagnetic force due to the inclination of the electrode relative to horizontally disposed source material. Also disclosed in the '369 patent is the step of incorporating a "pinch gas", also commonly known as a working gas, into the working gas. A working gas is typically, an inert gas that acts to shield one or more of the electrodes, and more importantly, is ionized to establish and sustain an arc. Araya discloses using a pinch gas of Argon mixed with Hydrogen, Nitrogen, or Oxygen in order to increase the amount of heat produced.
However, U.S. Pat. No. 5,514,349 to Parker, et al. for "A SYSTEM FOR MAKING POLYCRYSTALLINED MATERIALS," disclosed a disadvantage associated with the practice of using Oxygen as a dissociable gas. Oxygen is not preferably used in a working gas because of the resulting erosion of the non-consumable electrode. The '349 patent also discloses a non-consumable electrode inclined at an angle to the source, or evaporative, material to create an elongated arc plasma tail flame. By including Nitrogen, Hydrogen or both into the working gas, the plasma tail flame temperature is increased, which will result in a more complete reaction of the evaporative material with a reaction gas such as Oxygen, Nitrogen, Helium, Air or a combination of these gases. The presence of a reaction gas enables the source material to form nano-sized compounds. For example, if the source material is Titanium which is evaporated and then exposed to a reaction gas containing some concentration of Oxygen, Titanium (TiO.sub.2) polycrystalline materials may result.
The above-mentioned patents employ continuous gas injection into a vaporization chamber, which makes it necessary to include a continuous vacuum pumping system for gas circulation. Furthermore, as gas is injected into the vaporization chamber, the chamber pressure will increase. The dynamic gas injection and gas circulation will require a more sophisticated system control process for operation, increasing the complexity of this system. Although productivity has been enhanced by the above-mentioned techniques, these gas phase condensation processes still involve a great deal of technical complexity. Furthermore, gas circulation requires the addition of gas filters and valves which will require maintenance and cleaning after a period of operation, resulting in system downtime and still more system complexity. Also, continuous gas injection into the evaporation chamber, and the subsequent release of the gas from the chamber, will consume a great quantity of gas, which leads to higher operation costs.
A simplified method for evaporating materials is disclosed in U.S. Pat. No. 5,096,558 to Ehrich for "METHOD AND APPARATUS FOR EVAPORATING MATERIAL IN VACUUM." The '558 patent discloses a technique for evaporation of materials at very low pressures (10.sup.-4 millibars to 10.sup.-2 millibars) for the purpose of coating surfaces. The method and apparatus disclosed places much emphasis on the benefit of anode evaporation. The materials disclosed for evaporation by this method are high vapor pressure, low melting temperature materials. Although this technique works well for low-melting temperature materials, anode evaporation does not create sufficiently hot cathode spots to evaporate low vapor pressure, high temperature materials. Furthermore, the operating pressure range disclosed is too low to form particles having polycrystalline structures.
An arc plasma is a low resistance electrical conductor consisting of a high-density mixture of ionized atoms or molecules, electrons, and neutral species. Because a substantial current passes between the electrodes, typically tens to hundreds of amperes, a stable arc requires a high density of conducting molecules. If the chamber pressure is above several tens of Torr, the working gases, which may be either inert or active, will act as the current transfer medium. This type of arc evaporation may be referred to as high-pressure arc evaporation. High-pressure arc produces greater thermal power and requires a higher current, and is able to raise a non-consumable electrode to a high temperature, typically thousands of degrees centigrade. A common use of high-pressure arc evaporation is in the field of thermal spray surface coating.
Low-pressure arc evaporation methods operate in a chamber at a pressure below about 10 Torr. In a low-pressure arc evaporation system, the arc is sustained by substantial evaporation of electrode materials. The arc is initiated by some means such as a high frequency ignition or simple contact ignition. Alternatively, low-pressure arc evaporation may also be referred to as vacuum arc evaporation. Basically, a vacuum arc is sustained by the vapor emitted from a consumable electrode which may be either the anode or cathode. A true vacuum arc uses the evaporated species as the primary conductor for the arc. The consumable electrode provides the medium for the current path, which makes vacuum arc technology suitable for a wide spectrum of applications for vacuum thin-film coating, however this technology is not suitable for production of polycrystalline particles. Accordingly, there exists a need for a simple, efficient technique for forming polycrystalline particles.